1. Technical Field
The present invention relates to photolithographic lens systems and, more specifically, to a method and apparatus for determining projection lens transmission distribution while separating contributions of the illumination source aperture uniformity from the projection lens pupil transmission distribution in a photolithographic lens system.
2. Background Art
Optical photolithography has been widely used in the semiconductor industry in connection with the formation of a wide range of structures in integrated circuit (IC) chips. Complex forms of pupil illumination patterns have become increasingly common for the purpose of improving resolution or depth of focus. The ability to measure and verify the correct distribution of illumination in the pupil plane has become increasingly critical.
Uniformity of the illumination at the wafer surface is needed so that the same exposure of photoresist or other light sensitive films is consistently achieved across the entire exposure field. The degree of partial incoherency of the illumination, or more generally the distribution of pupil illumination, must also be constant across the entire exposure field. As tolerances of the printed lithographic patterns become increasingly tight, the requirement that the pupil illumination distribution not vary across the exposure field becomes increasingly important.
Various illumination systems for lithographic lenses have been developed, including those that create complex patterns of pupil illumination to enhance lithographic resolution and/or depth of focus. Illumination patterns, such as dipole, quadrupole, and annular shapes, have been developed to improve the resolution and depth of focus of the image formation. Some of these illumination patterns are particularly suited to enhancing the lithographic performance of specific mask patterns that are exposed on the stepper. When conventional partially coherent illumination is used, the center of the pupil is illuminated uniformly out to a prescribed fraction of the pupil size. In the case of both conventional partially coherent illumination and the more complex off-axis illumination patterns, the consistency of the illumination pattern at every position in the exposure field is critical.
As dimensions of IC components are continually reduced, and as the wavelength of light used in photolithography is reduced, it becomes increasingly important to be able to determine the sources of non-uniformity in illumination at the wafer plane. As the lens systems used for photolithography become more complex, it becomes more important to be able to isolate portions of the systems in seeking to determine such sources of non-uniformity.
The quality and uniformity of the illumination at the wafer plane can be analyzed and characterized by a variety of techniques, including wafer-plane power meters, analysis of photoresist or other light-sensitive films, etc. In the past, the pupil illumination has been measured by using either a single, relatively large (one to a few millimeters), aperture in the plane of the photomask, or a plurality of pinholes in an array. In both cases, the aperture functions as a pinhole camera and projects a geometrical image of the pupil illumination pattern.
However, in the past it has not been possible to separate contributions to non-uniformity originating in the illumination source from contributions to non-uniformity originating in the projection lens system.
Therefore, a need exists for a system that can efficiently obtain quantitative measurements of the illumination pattern at the wafer plane of the photolithographic lens system, while separating contributions to non-uniformity originating in the illumination source from contributions to non-uniformity originating in the projection lens system.